Semiconductor packages are becoming stronger EMI (electromagnetic interference) emitters within an electronic system because chip operating frequencies and power are increasing. In order to ensure compliance to international standards for EMC (electromagnetic compatibility), system designers must work with electronic packaging designers of the various subsystems: power supplies, motors, cable interconnection, printed circuit board and chip modules. These several designers must identify the strong EMI emitters and proactively apply EMI mitigation practices. One area of increasing focus is the high speed, highly integrated semiconductor package. On chip currents and voltages create magnetic and electric fields that travel through free space. These stray electro magnetic (EM) fields can adversely interact with other assemblies within the system and also, can leak through system cabinet seams, openings and vents. EM emissions from chips can be intercepted by an electrically conductive the heat spreader or lid that is typically adhesively bonded directly to the back side of a flip chip and the heat spreader is electrically grounded.
One practice has been to use metal clips that are installed on the finished module or after the module has been attached to a printed circuit card or board. The clip electrically and mechanically links the electrically conductive heat spreader to a ground pad either on the chip substrate or directly to the card, i.e., the chip package. Clip installation is difficult to automate and also has the potential to create metal debris from scraping. Since the clips protrude beyond the standardized physical outline of the package, forethought might be required during the card or board design to allow extra space for installation. A more preferred grounding solution is one that can be easily incorporated during the chip package manufacturing. Solder interconnection has been evaluated and found to be sensitive to fatigue cracking. Solder connects the underside of the electrically conductive heat spreader to ground pads on the substrate.
These connections are short in length and exposed to package shear and tensile cyclic stresses from heating and cooling that eventually result in cracked solder joints. An alternative to solder is electrically conductive adhesive (ECA) joining which provides a more strain/stress tolerant connection. However, ECAs require noble metal finishes for low contact resistance that is stable through environmental stress. Non noble finishes such as nickel and copper might provide low contact resistance just after bonding but are subject to continued oxidation and hydration that leads to high resistances in the several ohm range. Further, ECA cure chemistry has been known to adversely interfere with the cure chemistry of sealband adhesives that are used to bond the electrically conductive heat spreader to the substrate. ECA joints are vulnerable to degradation during impact shock loading.